The present invention relates generally to inkjet ink compositions and methods of for making such compositions. More particularly, the present invention relates to lightfade-stable magenta inkjet ink compositions displaying both high lightfastness and chroma.
An inkjet image is formed when a precise pattern of ink dots is ejected from a drop generating device known as a xe2x80x9cprint headxe2x80x9d onto a printing medium. A typical inkjet print head has an array of precisely formed nozzles located on a nozzle plate and attached to an inkjet print head base. The base incorporates an array of firing chambers that receive liquid ink (colorants dissolved or dispersed in a solvent) through fluid communication with one or more ink reservoirs. Each chamber has a thin film resistor, known as xe2x80x9cfiring resistorxe2x80x9d located opposite the nozzle so ink can collect between the firing resistor and the nozzle. In particular, each resistor element, which is typically a pad of a resistive material, measures about 35 umxc3x9735 um. The print head is held and protected by an outer packaging referred to as a print cartridge, i.e., inkjet pen.
Upon energizing of a particular resistor element, a droplet of ink is expelled through the nozzle toward the print medium. The firing of ink droplets is typically under the control of a microprocessor, which conveys signals through electrical traces to the resistor elements. Thus the formation of alphanumeric and other characters on the print medium is possible. Both the initial quality and permanence of the image produced by an ink composition when applied to a given print medium or substrate are important. Various image characteristics determine overall image quality and permanence and may be manipulated in order to achieve desired results suitable to a specific application. Thus, the image quality attributes include such features as the color vividness and saturation, grain, quality of half-toning, and correctness of the color reproduction. The color vividness can be characterized by the chroma (C*) number of the CIELAB color system, and can readily be measured by a spectrophotometer. Color vividness substantially depends upon the paper on which the image is printed. Normally, special ink-jet photo papers enhance color vividness of ink-jet inks. On the other hand, office papers pose a greater challenge in providing the color vividness required. It is quite common therefore to characterize the color vividness of a given ink set by the chroma on a plain office paper, for example, on TM HP Printing(trademark) Paper.
On the other hand, the image permanence attributes include the stability of images to light (lightfastness), stability to humidity (humidfastness), as well as the stability to atmospheric gases and pollutants (airfastness). Out of those three attributes, lightfastness has attracted the most attention recently. Lightfade has been a well documented phenomenon for color photographs; see, for example, the monograph: Wilhelm, H. G., The permanence and care of color photographs: traditional and digital color prints, color negatives, slides, and motion pictures, Grinnell, Iowa, U.S.A.: Preservation Pub. Co., 1993, which is incorporated herein by reference. It is quite common to measure the lightfastness of photographs in years to failure. According to Wilhelm, the failure occurs after a specific percentage of loss in optical density. For example, for magenta, 25% loss of the green-filter optical density represents the failure. In Wilhelm""s monograph, it is suggested that the average exposure of photographs indoors is equal to 450 Lux per 12 hour day, which is equivalent to 1971 klux-hour per year. Taking into account this number, one can estimate the approximate time to failure, as based on the total light exposure of the sample in luxes required to reach the failure end point.
In the art of formulation of ink-jet inks, it is well established that the dyes that produce brighter colors tend to show poor lightfastness, and vice versa, that is, dyes having better lightfastness tend to be dull. Thus, common ink-jet dyes, such as, Acid Red 52 (Magenta), Acid Blue 9 (Cyan), and Acid Yellow 23 (Yellow) provide excellent color vividness, but extremely poor lightfastness. On the other hand, dyes containing a metal in their structure (called below metallized dyes), such as Reactive Red 23 or Direct Blue 199 show low or intermediate color vividness combined with very high lightfastness. Finally, there is a group of dyes that combines intermediate lightfastness and color vividness; among those dyes one can mention, for example, azo dyes such as Reactive Red 180.
A known approach in formulation of ink-jet inks involves blending several dyes together, so that one dye in the blend provides the color vividness, while the other improves lightfastness. This approach has been described, for example, in patents U.S. Pat. No. 5,536,306 by Johnson et al., and U.S. Pat. No. 5,772,742 by Wang. Very little is known about the interaction of the dyes in the blends, in terms of the lightfastness of the blend compared to that of the individual dyes. It is known for example, that some dyes show autocatalytic fade in the presence of the other dyes; for example, M377 dye produced by Ilford, Switzerland, fades quicker in presence of copper phthalocyanines, as reported by the manufacturer. In many other cases, a moderate stabilization effect is observed; thus, the dye fade in HP 970C magenta as a primary color is faster than in the secondary colors and in the composite black on HP Premium Plus Photo Paper. These effects are considered to be minor, and as the first approximation it is believed that the lightfastness of the mixtures is additive. Thus, when a more stable dye is chosen for the blend, the lightfastness of the blend is expected to increase.
Despite the considerable efforts in optimizing the lightfastness and color vividness performances of ink-jet inks, the need remains in further improvement upon both the color vividness and lightfastness.
Accordingly, the present invention provides a magenta inkjet ink formulation that includes a mixture of an effective amount of a rhodamine magenta dye and an effective amount of a metalized magenta dye in an ink vehicle. Such an inkjet ink formulation has been found to provide superior chroma and lightfastness characteristics which are synergistic (i.e. exceed the expected performance for such a combination). In one aspect, the formulation achieves a chroma of at least about 57 on HP Printing Paper and a lightfastness of at least about 5 years on HP Premium Plus Photo Paper. In a more detailed aspect, the formulation achieves a chroma of 60 on HP Printing Paper and lightfastness of at least about 10 years on HP Premium Plus Photo Paper.
A wide variety of rhodamine dyes may be used in the present invention. However, in one aspect, the rhodamine dye may be a member selected from the group consisting essentially of Acid Red 52, Acid Red 289, Acid Red 388, and mixtures thereof. In another aspect, the rhodamine dye may be Acid Red 289. In still another aspect the rhodamine dye can be Acid Red 52. In yet another aspect, the rhodamine dye mixture can be a mixture of Acid Red 52 and Acid Red 289.
A number of metalized magenta dyes may be used in the inkjet ink formulation of the present invention. However, in one aspect, the metalized magenta dye may be a copper containing dye. In another aspect, the metalized magenta dye may be a nickel containing dye. In yet another aspect, the copper containing dye may be a member selected from the group consisting essentially of: Reactive Red 23, pacified Reactive Red 23, and mixtures thereof. In a further aspect, the copper containing dye may be pacified Reactive Red 23.
The amount of rhodamine and metalized magenta dyes used in the present invention may be any amount sufficient to achieve an inkjet ink with the increased chroma and lightfastness characteristics achieved by the present invention. However, in one aspect of the present invention, the amount of rhodamine dye, or rhodamine dye blend is sufficient to achieve an absorbance from 0.03 to 0.20 at a 1:10000 dilution as measured at the peak absorbance wavelength, which is located between 500 and 600 nm. In another aspect, the amount of metalized magenta dye is sufficient to achieve an absorbance from about 0.03 to about 0.20 at a 1:10000 dilution as measured at the peak absorbance wavelength, which is located between 500 and 600 nm.
The present invention additionally encompasses a method of making an inkjet ink formulation that achieves superior chroma and lightfastness characteristics. In one aspect, such a method includes the step of mixing an effective amount of a rhodamine dye and an effective amount of a metalized magenta dye in an ink vehicle. The specific types of each component as described herein may be used in a variety of combinations to form such an inkjet ink.
The present invention also includes a method of making an inkjet image having high lightfastness and chroma characteristics. In one aspect, the formulation achieves a chroma of at least about 57 on HP Printing Paper and a lightfastness of at least about 5 years on HP Premium Plus Photo Paper. In a more detailed aspect, the formulation achieves a chroma of about 60 on HP Printing Paper and lightfastness of at least about 10 years on HP Premium Plus Photo Paper.
There has thus been outlined, rather broadly, the more important features of the invention so that the detailed description thereof that follows may be better understood, and so that the present contribution to the art may be better appreciated. Other features of the present invention will become clearer from the following detailed description of the invention, taken with the accompanying claims, or may be learned by the practice of the invention.